The Earth Science Picture of the Day (EPOD) highlights the diverse processes and phenomena which shape our planet and our lives. EPOD will collect and archive photos, imagery, graphics, and artwork with short explanatory captions and links exemplifying features within the Earth system. The community is invited to contribute digital imagery, short captions and relevant links.

Accretionary Lapilli

Little rock balls, all over the place! They’re called accretionary lapilli (singular lapillus) and are sometimes formed during volcanic eruptions through water or water-saturated ground (phreatomagmatic eruptions). When molten bits of volcanic ash are lofted into the air during an eruption, they stick together as they tumble and fall. Slowly they accumulate more and more ash particles. Different parts of the eruption column may have slightly different compositions so each lapillus grows by adding distinct concentric layers. Eventually, they fall from the plume to the ground, and thus are somewhat analogous to hailstones. Accretionary lapilli can also form in the plume from an asteroid impact and may have been found on Mars.

The growth process may proceed along several different paths, but all seem to involve the presences of rain where surface tension plays a role in bringing particles together. Electrostatic attraction is also thought to promote their formation. If the loosely bound lapillus becomes firmly cemented before reaching the ground, it’ll survive impact. Some lapilli flatten or disintegrate on impact. The cementation process isn’t well understood but probably involved a “glue” that’s the product of chemical reactions between sulfuric acid and the volcanic rock.

These lapilli were found on the flank of the Darwin Volcano in the Galapagos Islands. They’re hard, can’t be crushed between the fingers, yet have much lower density than the rock they’re made of – pyroxene and plagioclase. With a porosity of about 50 percent, there’s a considerable amount of air in gaps between the constituent glassy grains, something not unexpected in view of how they form.